Transmission electron microscopic studies were made to evaluate and quantify the severity of ischemic injury in the cellular components of human aortic valves that were subjected to various time delays before harvesting and cooling in order to prepare them for cryopreservation and use as human cardiac valve allografts. The results obtained show no morphologic injury in valves with harvest-related warm ischemic times less than 2 hours and minimal irreversible cellular injury in valves exposed to 12 hours or less of warm ischemia. If cellular viability is critical to homograft durability, harvest-related warm ischemia may need to be restricted to 12 hours.